The intra-ocular pressure (IOP), i.e., the pressure within the eyeball, of a person is one of the most important parameters indicating the health status of the person's eye. Various eye diseases cause the IOP to be higher or lower than normal, but it is mainly elevated when the patient is suffering from glaucoma. Glaucoma is an extremely common condition afflicting about 2% of the population over 40 years of age and is one of the major causes of blindness in the world. This disease has no symptoms and is usually diagnosed by tonometry measuring the IOP of the subject. Tonometry is therefore a routine procedure in all eye examinations, especially those of adults.
There are various types of tonometers for measuring the IOP. One type, called an "indentation tonometer", or "impression tonometer", measures the IOP by measuring a deformation produced in the subject's cornea when a constant force is applied. However, the more common type is the "applanation tonometer", which flattens the cornea and measures the force applied. The commonest and most reliable tonometer used at the present time is called the Goldmann applanation tonometer, in which a flat plate is pressed against the subject's cornea, and the area of applanation is viewed by means of a split-lamp and microscope until the diameter of applanation is found to be 3.06 mm. Thus, it was found by Goldmann that at an applanation diameter of 3.06 mm (or an applanation area of 7.35 mm.sup.2), the force required to distort the cornea from its convex shape to a flat shape counterbalances the surface tension effect of the tear of the subject, such that when using this applanation diameter, the force in grams multiplied by "10" is directly converted to the IOP in mm of Hg.
In the Goldmann tonometer, the applanation area is measured by optically splitting it into two halves by a biprism, one half being displaced 3.06 mm relative to the other. A fluorescent solution is first applied to the eye to form a ring which is seen as two semi-circles. A dial is then manually rotated to apply a flattening force to the subject's cornea. When the two semi-circular rings touch, the position of the dial, calibrated in mm Hg, indicates the force required to produce an applanation diameter of 3.06 mm.
A problem with applanation tonometers in general, and the Goldmann applanation tonometer in particular, is the relatively long contact time between the flat plate and the subject's cornea during the measurement. Not only is a long contact time very unpleasant to the subject, but any movement of the subject's eye during the contact time may require restarting the procedure. This long duration of contact generally requires anesthetizing children, and sometimes adults, in order to make the IOP measurements.
Other drawbacks in the use of the existing Goldmann applanation tonometer for making IOP measurements are the dependence or accuracy on the expertise of the person making the measurements, and the large size and bulkiness of the apparatus used for making such measurements.